1. Field of the invention
The invention relates to a controlled release liquid urea-formaldehyde fertilizer which has a nitrogen level greater than about 31%.
2. Description of Related Art
Urea-formaldehyde based liquid fertilizers have been used for some time to provide nitrogen to the soil. It is desirable that such fertilizers be stable and provide a steady, evenly supplied, amount of nitrogen.
In the past, long term stability of high nitrogen (around 30%) liquid urea-formaldehyde fertilizers was achieved by forming either a high percentage (more than 30%) of cyclic triazone structures or by condensing the urea-formaldehyde resin into small urea-formaldehyde polymer chains.
Several patents issued to Hawkins describe the preparation of urea-formaldehyde resins having high triazone contents. U.S. Pat. No. 4,554,005 describes a reaction that produces at least about 30% triazone and has a preferred urea, formaldehyde, ammonia ratio of 1.2/1.0/0.28. U.S. Pat. No. 4,599,102 describes a reaction that produces at least about 30% triazone and has a urea, formaldehyde, ammonia ratio of 1.2/1.0/0.5. Both of these resins have a high percentage of ammonia. U.S. Pat. No. 4,776,879 describes a reaction that produces at least about 75% triazone in water insoluble forms. This material is then crystallized out and redissolved at low solids levels for use. U.S. Pat. No. 4,778,510 describes a reaction that produces at least about 48% triazone. Nitrogen release from triazone is extremely slow, therefore, methylolated urea is preferred. The nitrogen is the useful part of the fertilizer to the plant and thus the higher the % of nitrogen, the more efficient the fertilizer.
Other patents describe condensing the resin into small chains. U.S. Pat. No. 4,781,749 to Moore reacts 1.5 to 2.5 mols formaldehyde per mole of urea in the presence of ammonium compounds such as ammonia. This initial mole ratio is below the initial mole ratio of 5 to 4 mols formaldehyde per mole of urea of the present invention. The pH is maintained at near neutral conditions (6.9-8.5) throughout the reaction. Condensed UF chains have lower solubility than methylolated ureas and could continue to advance, leading to extremely slow release.
U.S. Pat. No. 3,970,625 to Moore et al. describes a process for preparing urea-formaldehyde concentrates for use as slow release fertilizers or as adhesives. Urea and formaldehyde are mixed in a molar ratio of 1/4.4-7.3 with no more than 0.015 wt. % of ammonia present in the urea. The pH is adjusted to 8.8-9.5 and the mixture is heated to 50-60.degree. C. for 30-60 minutes. Thereafter, water is removed by distillation under reduced pressure until solids comprise 60-90% of the remaining residue. For producing fertilizers, the residue is heated for another 48 hours at a temperature of 45-50.degree. C.
U.S. Pat. No. 5,449,394 to Moore relates to liquid non-polymeric controlled-release nitrogen plant food compositions containing the condensation products of one part ammonia, two parts urea and three parts formaldehyde at a base buffered pH slightly above 7. The reaction is accomplished at a temperature of about 100.degree. C. for 30-300 minutes. Water may be removed by evaporation until the nitrogen content of the formulation is between 20 and 30%. The solution is cooled before polymerization producing chains of more than 3 urea moieties can occur.
It is desirable to have a method of making a stable urea-formaldehyde resin suitable for fertilizer use that uses significantly less triazone and no condensation, and has a higher nitrogen concentration.